The lithography printing process is based on the general principle that ink and water are immiscible. In conventional wet lithography ink as well as water are applied simultaneously to the plate surface. In positive working lithographic printing plates the hydrophobic or oleophilic areas of the image, formed following exposure and development of the plate, will accept ink, whereas the hydrophilic or oleophobic non image areas, the background revealed following exposure and development, will accept water. The ink on the image is then transferred to the surface to be printed, for example paper, via an intermediate rubber blanket (offset printing).
In general lithographic printing plate precursors are formed by the application of a radiation sensitive coating to an anodised aluminium substrate. So called conventional printing plates have coatings that are sensitive to UV radiation, in positive working lithographic plates the solubility of the coating increases on imagewise exposure and therefore can be removed by the developer during the post exposure development step. In negative working plates the coating is insolubilized on imagewise exposure and the unexposed areas would be removed during development.
For many years conventional UV sensitive positive working lithographic plates have been based on the fact that the dissolution rate of novolak resins by aqueous alkaline solutions is strongly inhibited by diazonaphthaquinone (DNQ) sulphonates. This inhibition of dissolution is caused by the formation of a very stable hydrogen bonded matrix between the novolak hydroxy groups and the DNQ sulphonate groups (Arnost Reiser, Journal of Imaging Science and Technology, Volume 42, Number 1, January/February 1998, p. 15-22).
When exposed to UV radiation the photo-decomposition of the DNQ structure to the corresponding indene carboxylic acid, a reaction known as the Wolff rearrangement which is fast and highly exothermic, creates a high intensity heat spike which effectively releases the novolak from the hydrogen bonded matrix, and allows its penetration and dissolution by the aqueous alkaline developer. Due also to the formation of the readily soluble carboxylic by-products, this increase in dissolution rate can be as much as three orders of magnitude, leading to a very good image discrimination.
Recently, the above mentioned phenomenon of the inhibition of dissolution of novolak resins has been used with very good success in the compositions of lithographic printing plates precursors which can be used together with new generations of exposure technologies. Following developments in the field of digital and laser imaging, the so-called computer-to-plate or CTP technologies, the printing and graphics arts industries now require printing plates which can be efficiently exposed using these new technologies, known as direct laser addressable printing plate precursors.
Within the different types of direct laser addressable exposure equipment technologies (CTP technologies) the most widespread development has been in exposure equipment using lasers diodes emitting light in the near infrared (IR) wavelength region of 780-850 nm. These systems have come to be known as Thermal systems.
Thermal plate precursors for use with Thermal CTP equipment fall into two distinct categories, negative working and positive working.
In negative working thermal plate precursors IR absorbing compounds are used in conjunction with photo-acid generators (radiation induced decomposition of latent Brönsted acids) to insolubilize the binder polymer. The energy delivered by the laser is insufficient to fully complete the reaction and insolubilize the composition, and therefore the reaction is completed by a heating step prior to development of the precursor. These precursors are known as negative working pre-heat thermal plates.
In positive working thermal plate precursor compositions, the laser light is converted to heat by the IR absorbing compound and this heat is used, analogous to the conventional UV sensitive DNQ sulphonate/novolak compositions, to directly break the dissolution inhibiting hydrogen bonds formed between the binder polymer and the IR dye absorber, itself acting as an inhibitor, and any other additional inhibitor.
In practice the hydrogen bonds generated between the binder polymer, such as a novolak resin, and any non IR sensitive inhibitor can also be broken by the heat produced directly from the laser, but such a plate precursor would need a very high energy to bring about this change and would not be commercially viable.
In thermal positive plate precursors of the IR dye/novolak type compositions the IR dye thus has a dual function, firstly as the light to heat converting material, and secondly as the (co-)inhibitor of dissolution for the novolak resin. The IR dye can be described as imparting both the required IR sensitivity and the dissolution inhibition effect at the same time.
Following the processes of coating and drying the precursor during line manufacture, thermal plates need to undergo a process of stabilization in order to achieve the formation of the hydrogen bonded network which imparts the needed insolubility characteristics of the coating. However, due to the weak nature of the hydrogen bond forming capacity of typical IR absorbing dyes, the thermal printing plate precursors manufactured using coating compositions containing a blend of IR dye and novolak resin form only a weak matrix, and therefore this stabilization process proceeds very slowly at ambient temperatures (in comparison with DNQ/novolak systems). In order to avoid long storage times at ambient temperatures, the process duration time can be accelerated by a period of bulk storage of the precursors in conditioning ovens at controlled elevated temperature and relative humidity.
EP 0823327 discloses positive lithographic printing plate precursors comprising photosensitive compositions showing a difference in solubility in alkaline developer between exposed and unexposed portions, the composition comprising a photo-thermal conversion material and a high molecular compound, of which the solubility in an alkali developer is changeable by a change other than a chemical change. Many of the examples, such as Examples 1-10 and 74-77, disclose photosensitive compositions comprising phenolic resins/novolaks which are coated onto aluminium plates with a film thickness of 2.4 g/m2, dried and then subjected to stabilization at 55° C.
EP 1024958 discloses a method of manufacturing a positive no preheat thermal sensitive lithographic plate precursor which compromises a coating composition containing a phenolic resin on a substrate, the drying of the composition and the subsequent heat treatment of the coated substrate, wherein the heat treatment is carried out for at least 4 hours, and preferably for at least 48 hours, at a temperature in the range 40-90° C., preferably at least 50° C. and not in excess of 60° C. In the description the applicants state that they believe that if the elected temperature is too low then the time taken for the formation of the stable network structure will be too long to be of practical use. The formulations contain blends of phenolic resins and infrared absorbing compounds. The dry film coating weights of the compositions on the substrate is in the range of 2.0-2.5 g/m2. Although the method is useful for providing stable and consistent lithographic plate precursors, there are penalties in increased costs and production time through the need for an extra manufacturing process (in practice it is known that even at elevated temperatures this process time can be in excess of 10 days).
WO 02/11984 discloses a composition for a positive no preheat thermal sensitive lithographic plate precursor whose coating composition comprises of a hydroxyl group containing polymer (for example a novolak resin) and a process for the method of manufacture wherein the coating weight of the composition on the substrate is less than 1.1 g/m2 preferably no more than 0.9 g/m2. The patent application concerns a method to avoid the process conditioning step of heat-treating the plate precursor following coating and drying of the precursor on line. However the use of such low coat weights, in comparison with commonly used coating weights, can lead to a significant reduction in the durability of the plate, as for example in the useful print life of the plate and resistance to press room chemicals.
If such plate precursors are supplied to the end-user customers before the completion of the stabilization process then the process will continue at the customers premises leading to an unacceptable adjustments having to be made by the customer in the exposure and development process parameters. On the other hand, if the plate manufacturer does not precisely control this extra conditioning process then it can add considerably to the variation in the same finished product characteristics (plate sensitivity and development parameters). There are also obvious penalties for the manufacturer in terms of increased costs due to energy consumption, increased manufacturing lead times and complexity of supply logistics.
Despite the progress that has already been made in attempting to provide stable and consistent plate precursors there is still a need for compositions which do not require the additional problematic conditioning process in order to provide a stable product.
In the above cited patents, the formulations of the precursor coating compositions use the IR dye in a blend or admixture with the novolak resin, but it is also known that the infrared absorbing compound can take the form of a pendent chromophore group attached to the polymer backbone.
U.S. Pat. No. 6,124,425 discloses thermally reactive near infrared absorption polymer coatings, methods for preparing and methods for use. The patent teaches the preparation of both negative and positive working type sensitive polymers and the preparation of lithographic plates containing the polymers.
WO 01/94123 teaches the preparation and use of polymers containing a cyanine dye attached to the polymer backbone, the polymer additionally comprises o-quinonediazide groups attached to the same backbone as an extra inhibitor of dissolution.
EP 1186955 discloses the use of film forming polymers containing an infrared chromophore in the preparation of masks, mask precursors, electronic parts, and their precursors. The patent is not concerned with the preparation of lithographic plates.
EP 1297950 discloses polymers for use in the preparation of lithographic plates comprising a chromophoric moiety which absorbs visible light in the wavelength region of 400 to 780 nm. The patent is concerned with the attachment of colorant dyes to the polymer backbone to avoid staining of the lithographic plate substrate.
The above cited patents applications are concerned with the problems of multi-component systems in manufacture or dye migration thereafter, but not with the post manufacturing conditioning process. In these patent applications all examples refer to the use of only one chromophore type attached to the polymer backbone.
It is the object of the present invention to overcome the disadvantages of thermal sensitive positive lithographic plate precursors that require a lengthy post manufacturing conditioning process at elevated temperature and which do not suffer a reduced latitude in their performance.